Why is the output with print! different from with println!, followed by an endless loop?

[p6dz]

// The output with println! is from 1 to 499.

fn main() {
    let mut i = 1;
    while i <= 1000 {
        if i == 500 { continue; }
        println!("{}", i); 
        i += 1;
    }
}

// The output with print! is from 1 to 283.

fn main() {
    let mut i = 1;
    while i <= 1000 {
        if i == 500 { continue; }
        print!("{} ", i); 
        i += 1;
    }
}

[xfix]

Stdout handle uses buffered output, currently using std::io::LineWriter which flushes the buffer whenever a newline is detected. println! prints a newline, print! as used here doesn't.

It's possible to manually force STDOUT to be flushed by calling stdout's flush method.

use std::io::{stdout, Write};

fn main() {
    let mut i = 1;
    while i <= 1000 {
        if i == 500 { continue; }
        print!("{} ", i);
        stdout().flush().unwrap();
        i += 1;
    }
}

[p6dz]

Thanks!
Besides detecting a newline, std::io::LineWriter also flushes the buffer whenever output exceeds 1024 bytes, is it right?

[riking]

You may also consider moving the flush to the termination condition:

if i == 500 {
    stdout().flush().unwrap();
    loop {}
}

[quinedot]

• LineWriter flushes when the buffer is full, but
• it has configurable size, and
• the current default of 1024 isn't documented so it's almost surely considered a changeable implementation detail, and
• Stdout using LineWriter is also an implementation detail subject to change